This invention relates to improvements in a fuel vapor storage canister serving as a fuel vapor treatment device for adsorbing fuel evaporated, for example, in a fuel tank of an automotive vehicle in order to combust the fuel in an engine during operation of the engine, fuel vapor adsorbent used in the fuel vapor storage canister, and a method of producing the fuel vapor adsorbent.
As is well known, a fuel vapor storage canister has been generally used as a fuel vapor treatment device in order to suppress release of fuel vapor generated within a fuel tank to atmospheric air in a gasoline-fueled automotive vehicle. This canister includes a casing formed thereinside a gas passage which is filled with a fuel vapor adsorbent such as activated carbon. Charge and purge ports for fuel vapor are communicated with one end of the gas passage, while an atmospheric port (drain port) for fuel vapor is communicated with the other end of the gas passage, thus accomplishing so-called charging. During stopping of the vehicle, fuel vapor generated from the fuel tank is introduced through the charge port into the canister and adsorbed by the adsorbent. During operation of an engine, atmospheric air is introduced through the atmospheric port to purge fuel vapor in the canister upon desorbing fuel vapor adsorbed in the adsorbent, and carries the purged fuel vapor to an intake system of the engine through the purge port so that the carried fuel vapor is combusted within the engine, thus accomplishing a so-called purging. By the desorption of fuel vapor under such purging, a fuel vapor adsorbing performance of the canister can be revived thereby allowing the adsorbent to repeatedly adsorb fuel vapor.
In the canister using the above-mentioned activated carbon, the adsorption of fuel vapor is carried out under so-called exothermic reaction so that the temperature within the canister rises with adsorption of fuel vapor thereby degrading the fuel vapor adsorbing performance of the canister. To the contrary, the desorption of fuel component (resulting from fuel vapor) which has been once adsorbed in the activated carbon is carried out under so-called endothermic reaction so that the temperature within the canister lowers with desorption of fuel component thereby degrading a fuel vapor desorbing performance of the canister. Such phenomena is well known.
Accordingly, a variety of conventional fuel vapor storage canisters have been proposed for the purpose of improving the above adsorbing and desorbing performances of the canisters.
Japanese Utility Model Publication No. 2-19570 proposes that the particle diameter of activated carbon becomes larger as the position of the activated carbon becomes nearer to the purge port of a canister. Japanese Patent Provisional Publication No. 2000-303917 proposes that activated carbon having relatively large particle diameters are disposed in a section near the charge and purge ports while activated carbon having relatively small particle diameters are disposed in a section near the atmospheric air port. Conversely, Japanese Patent Provisional Publication No. 2004-225550 proposes that activated carbon having relatively small particle diameters are disposed in a section near the charge and purge ports while activated carbon having relatively large particle diameters are disposed in a section near the atmospheric air port. Additionally, Japanese Patent Provisional Publication No. 2005-171797 proposes that at least one of particle diameter and pore diameter of activated carbon is positively changed at positions in flow direction of fuel vapor.